Substance use disorder is a condition in which the repeated use of a substance (such as alcohol, nicotine, prescription drugs, or illegal drugs) leads to significant problems or distress. These problems can include health issues, impaired social functioning, relationship problems, financial difficulties, and legal issues. The diagnosis of substance use disorder is based on a pattern of behavior that includes a combination of impaired control over substance use, continued use despite adverse consequences, and a strong desire or craving to use the substance. The severity of substance use disorder is typically determined by the number of symptoms present, with mild, moderate, and severe classifications.